The present invention relates to a DC motor driving device.
An example of a conventional DC motor driving device is shown in FIG. 1. In this device, a digital signal is employed as an input signal, and a DC motor is rotated in a forward direction or in a reverse direction according to the content of the input signal.
In FIG. 1, input terminals IN.sub.0 through IN.sub.2 are connected to inputs of a decoder 1. In the decoder 1, the three-bit (in this example) parallel digital signal is subjected to logical conversion to provide a six-bit logical output at output terminals CONT, A, B, C, D and E. The output terminal CONT is connected to a voltage control circuit 2 composed of NPN transistors 201 and 202, a reference voltage generating source 204, and a resistor 205. The transistors 201 and 202 are Darlington connected, and the base of the transistor 201 is connected to the emitter of the transistor 202. A supply voltage V.sub.cc is applied to the collectors of the transistors 201 and 202. The emitter of the transistor 201 forms the output terminal of the voltage control circuit 2. The resistor 205 is connected between the collector and the base of the transistor 202. The base of the transistor 202 is connected through a switch 203 to the reference voltage generating source 204. The switch 203 has a control terminal connected to the output terminal CONT of the decoder 1.
The output terminals A through E of the decoder 1 and the output terminal of the voltage control circuit 2 are connected to a motor driving circuit 3. The motor driving circuit 3 has first through fifth switch circuits 4 through 8.
The first switch circuit 4 includes NPN transistors 301 through 303 and a current source 304. The transistors 301 and 302 are Darlington connected. The base of the transistor 301 is connected to the emitter of the transistor 302. The collectors of the transistors 301 and 302 are connected to a power source line l coupled to the output terminal of the voltage control circuit 2. The current source 304 is connected between the collector and the base of the transistor 302, the base of which is grounded through the collector and the emitter of the transistor 303. The base of the transistor 303 is connected to the output terminal B of the decoder 1.
The second switch circuit 5 includes NPN transistors 305 through 307, and a current source 308, which are arranged in the same manner as the corresponding elements of the first switch circuit 4. The base of the transistor 307 is connected to the output terminal A of the decoder 1.
The third switch circuit 6 includes NPN transistors 309 through 311, resistors 312 and 313, and a current source 314. The collector of the transistor 310 is connected to the power source line l. The current source 314 is connected between the collector and the base of the transistor 310. The emitter of the transistor 310 is grounded through a series circuit of the resistors 312 and 313. The connecting point of the resistors 312 and 313 is connected to the base of the transistor 309, the emitter of which is grounded. The base of the transistor 310 is grounded through the collector and the emitter of the transistor 311. The base of the transistor 311 is connected to the output terminal E of the decoder 1.
The fourth switch circuit 7 is composed of NPN transistors 315 through 317, resistors 318 and 319, and a current source 320, which are arranged in the same manner as the corresponding elements of the third switch circuit 6. The base of the transistor 317 is connected to the output terminal D of the decoder 1.
The fifth switch circuit 8 includes NPN transistors 321 through 323, which are arranged in the same manner as respective components of the third switch circuit 6. The base of the transistor 323 is connected to the output terminal C of the decoder 1.
The motor driving circuit 3 thus constructed has three output terminals CMH, CMR and CMG. The emitter of the transistor 301 and the collector of the transistor 309 are connected to the output terminal CMH. The emitter of the transistor 305 and the collector of the transistor 315 are connected to the output terminal CMR. The collector of the transistor 321 is connected to the output terminal CMG. A DC motor 9 is connected between the output terminals CMH and CMR, and a series circuit of resistors 10, 11 and 12 is connected therebetween. The DC motor 9 has positive and negative terminals. The positive terminal is connected to the output terminal CMH. The connecting points of the resistors 10, 11 and 12, the negative terminal of the motor, and the output terminal CMG are connected to a constant rotation rate control circuit 13, which is constructed in the form of an integrated circuit.
In the conventional DC motor driving device thus constructed, a three-bit digital signal is applied through the input terminals IN.sub.0 through IN.sub.2 to the decoder 1. In the decoder 1, the three-bit digital signal is converted into a six-bit digital signal according to a suitable coding system in such a manner that conversion outputs are provided at the output terminals CONT, A, B, C, D and E according to the contents of the input digital signal. In driving the motor 9 in the forward direction with a constant voltage, the output terminals CONT, A C, and E are held at the high level and the output terminals B and D at the low level. As the output terminals B and D are at the low level and the output terminal CONT is at the high level, the switch 203 is turned on, and the reference voltage generating source 204 applies a voltage V.sub.ref to the base of the transistor 202. Therefore, the emitter voltage of the transistor 201, i.e., the output voltage V.sub.O of the voltage control circuit 2, can be represented by the following equation: EQU V.sub.O =V.sub.ref -V.sub.BE201 -V.sub.BE202, (1)
where V.sub.BE201 is the base-emitter voltage of the transistor 201 and V.sub.BE202 is the base-emitter voltage of the transistor 202.
The transistor 307 is rendered conductive by the high level signal from the output terminal A, whereupon current from the current source 308 flows in the transistor 307. Therefore, the base potential of the transistor 306 is made substantially equal to ground potential, and the transistors 306 and 305 are rendered nonconductive. Similarly, the transistors 321 and 309 are rendered nonconductive by the high level signals from the output terminals C and E. On the other hand, the transistor 303 is rendered nonconductive by the low level signal from the output terminal B, in which case current from the current source 304 flows to the base of the transistor 302 so that the transistors 302 and 301 are turned on. As the output voltage V.sub.O of the voltage control circuit 2 is supplied to the base of the transistor 302, a positive potential due to the output voltage V.sub.O is supplied from the emitter of the transistor 301 through the output terminal CMH to the positive polarity terminal of the motor 9. The transistor 317 is rendered nonconductive by the low-level signal from the output terminal D, whereupon current from the current source 320 flows to the base of the transistor 316 and the latter is rendered conductive. As a result, the output voltage V.sub.O of the voltage control circuit 2 is applied to the resistors 318 and 319, and the voltage divided by the resistors 318 and 319 is applied to the base of the transistor 315. Therefore, the transistor 315 is rendered conductive, and the collector potential of the transistor 315 becomes equal to ground potential. This potential is supplied through the output terminal CMR to the negative polarity terminal of the motor 9. Therefore, the voltage V.sub.PM applied across the terminal of the motor 9 in the normal polarity can be represented by the following equation: EQU V.sub.PM =V.sub.O -V.sub.BE302 -V.sub.BE301, (2)
where V.sub.BE302 is the base-emitter voltage of the transistor 302, and V.sub.BE301 is the base-emitter voltage of the transistor 301.
By substituting equation (1) for V.sub.O in equation (2), EQU V.sub.PM =V.sub.ref -V.sub.BE201 -V.sub.BE202 -V.sub.BE302 -V.sub.BE301. (3)
In rotating the motor in the opposite direction with a fixed voltage, the output terminals CONT, B, C and D of the decoder 1 are raised to the high level and the output terminals A and E are set to the low level. Because of the high level at the output terminal CONT, the output voltage V.sub.O of the voltage control circuit 2 has the value indicated by equation (1). Due to the low level at the output terminal A, the transistors 306 and 305 are rendered conductive, and the positive potential due to the output voltage V.sub.O is applied through the emitter of the transistor 305 and the output terminal CMR to the negative polarity terminal of the motor 9. On the other hand, the transistor 309 is rendered conductive by the low level signal at the output terminal E so that the collector potential of the transistor 309 becomes equal to ground potential. This potential is applied to the positive polarity terminal of the motor 9 through the output terminal CMH. Therefore, a voltage V.sub.MM applied across the terminals of the motor 9 in the opposite polarity can be represented by the following equation: EQU V.sub.MM =V.sub.ref -V.sub.BE201 -V.sub.BE202 -V.sub.BE306 -V.sub.VE305, (4)
where V.sub.BE306 is the base-emitter voltage of the transistor 306 and V.sub.BE305 is the base-emitter voltage of the transistor 305.
In holding the speed of the motor 9 to a fixed speed, the output terminals CONT, B and C of the decoder 1 are set to the low level while the output terminals A, D and E are raised to the high level. The switch 203 is turned off by the low level at the output terminal CONT. Therefore, the voltage V.sub.cc is applied to the base of the transistor 202 via resistor 205, and the transistors 202 and 201 are rendered conductive. In this operation, the output voltage V.sub.OC of the voltage control circuit 2 can be represented by the following equation: EQU V.sub.OC =V.sub.cc -V.sub.BE202 -V.sub.BE201. (5)
On the other hand, the transistors 302 and 301 are rendered conductive by the low level of the output terminal B, and the positive potential due to the output voltage V.sub.OC is applied through the output terminal CMH to the positive terminal of the motor 9. The transistor 321 is turned on by the low level of the output terminal C, whence the collector potential of the transistor 321 becomes equal to the ground potential. This potential is applied through the output terminal CMG to the input terminal S of the constant rotation control circuit 13. Therefore, a voltage V.sub.MC applied across the positive polarity terminal of the motor 9 and the input terminal S is as follows: EQU V.sub.MC =V.sub.OC -V.sub.BE302 -V.sub.VE301. (6)
By substituting equation (5) for V.sub.OC, EQU V.sub.MC =V.sub.cc -V.sub.BE202 -V.sub.BE201 -V.sub.BE302 -V.sub.BE301. (7)
In this operation, both of the transistors 305 and 315 are turned off, and therefore the output terminal CMR is placed in the open state. The circuit 13 detects the speed of the motor 9 and controls the voltage applied to the motor 9 according to the speed thus detected, thereby to maintain the speed of the motor 9 at a predetermined value.
In the conventional DC motor driving device described above, the motor driving voltage V.sub.PM or V.sub.MM used in driving the motor with a fixed voltage is lower, by the sum of base-emitter voltages of four transistors, than the reference voltage V.sub.ref, as is apparent from equation (3) or (4). The temperature coefficient of the base-emitter voltage V.sub.BE is about -2 mV/.degree.C., and therefore the temperature coefficient of of four base-emitter junctions is considerably large, about -8 mV/.degree.C. Accordingly, the accuracy of the voltage applied to the motor 9 is inherently low. Furthermore, the voltage V.sub.MC applied to the series circuit of the motor 9 and the control circuit 13 is lower, by the sum of the base-emitter voltages V.sub.BE of four transistors, than the supply voltage V.sub.cc, as indicated by equation (7). Therefore, sometimes a sufficiently high voltage cannot be applied to the series circuit of the motor 9 and the control circuit 13 if the supply voltage V.sub.cc drops, and hence the speed of the motor 9 cannot be correctly controlled.